Get astro turf then pour water/soap onto it.
For a box you might want to get PVC or that other fancy shmancy stuff (I FORGET THE NAME OF IT!) Or what I have is a cutting board like material.
if you mean the drop in?
then probally your local lowes or hardware store might have it.
this is mine, it works really well, i had to make it different because i have a tiny backyard
Old rail, best way to make one
i got it at my dads work, i think its just puckboard but i could be wrong, sorry im not too much of a help but it works really well if you can find some